A gas turbine typically compresses air and mixes it with fuel. The fuel, usually natural gas, is burned and the combusted gas expands to provide work. The gas turbine is coupled to and drives an electrical generator that produces electricity. After passing through the turbine, the hot gas passes to the gas turbine exhaust and exhaust stack into the atmosphere.
Gas-fired power plants are widely used, especially during peak or unscheduled demand, as they can be turned on and off within minutes. However, the many starts and stops along with high temperatures, pressure and velocity, puts additional stress on the duct and pipe systems. The increase in production demands the installation of highly engineered expansion joints to compensate for these stresses.
Typically, as shown in FIG. 3, gas turbine plan 1 has an expansion joint assembly 10 positioned between a turbine duct flange 5 and a diffuser duct 90, prior to the bypass exhaust stack 91, wherein the diffuser duct provides performance benefits to the turbine as a whole by expanding the exhaust gases to achieve optimum aerodynamic pressure recovery. Most turbine ducts run hot and are machined structures, while most diffuser ducts are lower cost fabricated casings that are internally insulated and relatively cold. Because of the thermal mismatch at this connection, an expansion joint is generally used to accommodate the large relative displacements between these components.
Ducting expansion joints are usually flexible connectors, which are designed to provide stress relief in ducting systems to absorb movement in the component parts of the ducting caused by thermal changes. Such ducting expansion joints also act as vibration isolators and compensate for minor misalignment of interconnecting ducts. Such expansion joint structures are also fabricated from a variety of metallic or non-metallic materials, including synthetic elastomers, fabrics, insulation materials, and plastics, depending on the designs. Such ducting expansion joints also find many applications, such as in smelters, refuse incineration and power generation plants, such as coal- or oil-fired plants, gas turbine plants, coal/oil/gas cogeneration plants, nuclear power plants, and also in pulp and paper plants and refineries, foundries, steel mills, etc.
Known expansion joints can take many forms, when relatively large axial, vertical, and lateral movements are expected.
U.S. Pat. No. 7,793,507 to Poccia, et al. teaches an expansion joint for use between a turbine duct and an exhaust duct. The expansion joint includes a flange attached to the turbine duct and a number of plates attached to the exhaust duct that extend towards the flange. The plates and the flange include a gap there between, the gap being narrower when the turbine duct is hot than when the turbine duct is cold. The expansion joint further includes a flexible element positioned between the turbine duct and the exhaust duct. The flexible element may be a nickel-based alloy. The flexible element is attached to the flange.
However, Poccia does not address the issue of flange stress caused by quick startup, and the gap design is not applicable to general gas turbine exhaust systems, which require a good seal within the joints to completely transfer the hot gas to exhaust stacks.
The expansion joint preferably provides a smooth aerodynamic transition between the ducts. For expansion joint assemblies of more than 4500 mm diameter, an angle rolled flange design has been produced to control thermal expansion of metal ducts. A standard design of such rolled angle flange is shown in FIG. 1, which shows a rolled angle heat absorption assembly made from A204 or A387 carbon steel of 6-8 mm thickness. The rolled angles or strips are welded together to create a dynamic system useful to control thermal expansion in duct systems for handling gas temperatures up to a maximum of 1004° F.
The two-step rolled carbon steel design allows for: 1) a reduction up to 50% of the temperature gradient; 2) a temperature reduction in both the lateral and axial directions; and 3) the deflected shape shown in dotted lines in FIG. 1 of the flange provides additional stress relief. The deflected shape is the result of the different degrees of radial growth that result from the thermal gradient that forms from the interior flow shield to the outermost damping area.
However, operating temperatures are now up to 1250° F. and above for new and more energy efficient gas turbines. The high operating temperature results in increased temperature gradients and associated increase in deflection stresses. In addition, the start-up time to full energy output of gas turbines has been reduced from 15 minutes to 6 minutes. The shorter start-up to maximum energy is faster than the heat absorption frame material can absorb and distribute the heat, thus, resulting in a larger thermal gradient and further compounding the dynamic balance between stress and maximum capacity for temperature. In addition to these changes, the number of start-up/shut down cycles has almost doubled. These frequent cold/hot/cold cycles introduce mechanical stresses in the form of fatigue. Such harsh operating conditions require special stainless steel materials like SS 347 or SS 321 to keep the dynamic thermal absorption system functional at these temperatures and stresses.
Stainless steel materials are difficult to produce into a flexible heat absorption flange with roll formed shapes as shown in FIG. 1, and therefore, stainless steel thermal absorption systems or flanges are fabricated as laser cut bands and arcs. Each wall or arc section is then welded into the step shape as shown in FIG. 2. This is especially the case for smaller diameter frames in the 3600 mm range. However, the welded design shown in FIG. 2 has become problematic for new turbine designs with the higher temperatures and reduced time to maximum output. The numerous welded areas have become the weak link in the design as a result of the additional stresses associated with the increased thermal gradient. This introduces an uncertainty factor for the welding, and requires that the welds be more tightly controlled and more vigorously inspected to insure integrity of the thermal absorption assembly, thus driving up manufacturing costs and also requiring the introduction of an increased safety factor leading to reduced cycle life.
U.S. Pat. No. 5,378,026 to Ninacs, et al. teaches a cylindrical flexible joint unit of circular cross-section having an inner annular flexible wall structure with an inner cylindrical sleeve, and an annular step connecting flange secured about the outer surface of the sleeve and forming an annular air space between the inner sleeve and the step connecting flange, wherein a rolled two-step shape design provides substantial stress level reduction. However, Ninacs' design retains a minimum of three welding points, and the geometry of the steps is such that the horizontal legs of the steps are grossly larger than the vertical legs, and thus unable to provide sufficient flex for the flange to be applicable in modern gas turbine operation, as the operational temperatures are now higher, up to 1250° F. Further, without the proper channel to expand or flex according to the heightened heated conditions, the sleeve will deform and most likely tear the back end of Ninacs' flange, exposing and losing insulation material due to gas velocity. Ninacs also utilizes Armco #409 stainless steel, a type of ferritic stainless steel that has inferior weld ability, low elongation factor, and is thus unsuitable for current gas turbine operation.
Therefore, there is a desire for an improved turbine expansion joint containing a dynamic or flexible flange for modern turbine design, such that the flange retains the rolled two-step design, reduces the amount of critical welds, and imparts minimal fatigue.